It takes 31 minutes for 7 people to paint 7 walls. How many minutes does it take 12 people to paint 12 walls?
Solution: Imagine that each person is assigned one wall, and all 7 people begin painting at the same time. Since everyone will finish painting their assigned wall after 31 minutes, it takes one person 31 minutes to paint one wall. If we have 12 people and 12 walls, we can again assign one wall to each person. Everyone will take 31 minutes to paint their assigned wall. In other words, it takes 31 minutes for 12 people to paint 12 walls.